move util.c to util.h

This allows single-source compilation and whole-program optimization using something like:

make CFLAGS='-flto -ffunction-sections -fdata-sections -Wl,--gc-sections'

Furthermore, move more compositor-agnostic utility functions/macros to util.h,
similar to the original dwm sources.

README.md

@@ -31,7 +31,7 @@ depend on functionality that is not in the current release of `wlroots`.
 The `wlroots-next` branch is built against the git version of [wlroots], which
 is unstable and changes frequently. PRs requiring functionality from the git
 version of `wlroots` should target this branch.
-  
+
 ### Building dwl
 dwl has the following dependencies:
 - libinput